A weblog for professionals in electrical, electronic, mechanical and software engineering with content provided by the members of the Long Island Consultant's Network.

Heat dissipation

December 15, 2012

Hurricane Sandy came through this area on October 29, 2012.
We had some house damage. Although nowhere near what was being shown in the
news, our electric power failed at 7:20 PM. Power stayed off for five days and
was then restored, only to fail again on November 8th in the midst of Nor'easter
Adele that brought a really heavy and wet snowfall. This second failure was repaired
on November 9th at 10:37 AM. All in all, we came though things relatively
unscathed.

I was up and awake when the second power failure occurred.
It was 2:15 AM. I checked the house's hall thermostat and saw that the temperature
reading was 68°F. I decided to record the temperature reading from time to time
while the power was off. This was the result:

December 22, 2011

A while back, we looked at how the specific heat of a heat sink mass can be used to examine the thermal rise time of an eight pound mass. We will now rescale that to a one pound mass, just to make these numbers a little easier to deal with, as we examine one additional issue regarding thermal rise, the time.

January 27, 2011

We have all kinds of information about finding maximum the temperatures that some particular things will get to, things like power supply heat sinks for example, but it may also be useful to look of how long will it take the temperature to get to that calculated extreme value.

With the specific heat of that particular something, we can make an estimate of the time constant of that something undergoing a thermal excursion:

Note: In this example, we look up the specific heat of aluminum at http://hyperphysics.phy-astr.gsu.edu/hbase/tables/sphtt.html where we find the value as 0.900 joules / °C / gram.

December 01, 2010

Imagine that you have this really complex circuit board which is going awry in some way when it gets hot and you can't seem to figure out which component is the culprit.

You might be able to isolate the problem to some one particular part if you could just heat up that one part by itself without also heating up the nearby surrounding parts.

You could try making a small tool and then do this:

If applying this very localized heat to a suspect part brings about the problem, you've found your culprit.

Note that although you could use the soldering iron itself to heat a suspected part, the soldering iron might dribble some molten solder, an event that would likely be bad news. The hot nail won't do that!

November 19, 2010

A product's cooling fan was rated only for 115 VAC service in a product that needed to switch between 115 VAC and 230 VAC. To accomplish this, I made the product's power transformer serve the fan as an autotransformer when 230V service was called for:At the lower line voltage, the fan was simply driven right from the line. At the higher line voltage, the fan acted as a load on the transformer's primary coils which served just fine as an autotransformer.

November 05, 2010

Switching power MOSFETs have a thermal runaway mechanism. A MOSFET's on-resistance rises with rising temperature and if the drain to source current doesn't materially change in response to that, a vicious cycle can occur. A rising Rdson leads to rising temperture which leads to rising Rdson which leads to rising temperature which leads to .......

You can test for this without having to burn anything by measuring MOSFET case temperature. Stasrt by measuring temperature at regular intervals as the devices get hot. Usually intervals of ten seconds are a good choice.

Once the MOSFETs have reached some high temperature, you turn off operating power AND you KEEP ON measuring tempertures as the MOSFETs cool down. Keep using the same time interval and keep on making the measurements until the MOSFET temperature has pretty much returned to the starting value.